Time of flight (TOF) mass spectrometers have developed into well established analytical instruments for identifying materials based on a distribution (spectrum) of charged particles differing in mass created by pulsed radiant energy or particle bombardment. A sample of material whose spectrum is sought is mounted as a target in an electric field. Bombardment with accelerated particles, such as perfect gas atoms or ions, or high intensity electromagnetic radiation, disrupts the molecules of the target to create a variety of charged particles--e.g., molecular ions, fragments, cations, and/or anions--hereinafter collectively referred to as ions. Once an ion of the sample material is created, it is accelerated in the electric field toward an electrode of opposite charge. A portion of accelerated ions is allowed to pass through an aperture in the attracting electrode and embark on a flight path which, through creation of an ambient vacuum, can be of extended length.
When the target sample receives a bombardment pulse, parcels of ions of like polarity but differing in mass are generated. Given that each ion creating collision imparts the same momentum EQU mv
where
m is mass and PA1 v is velocity, PA1 d is distance and PA1 t is time,
it follows that ions of greater mass have a lower velocity. Since velocity is EQU d/t
where
it follows that ions differing in mass within any single parcel will arrive at different times at a reference location along their common flight path. Stated another way, the original parcel of ions created by the bombardment pulse divides itself into partial parcels consisting of ions of the same mass and differing in mass from the ions of other partial parcels. By measuring and comparing the time of flight of partial parcels a spectrum of flight times can be identified which can them be mathematically translated into a mass spectrum unique to the sample material.
If all the ions in each partial parcel entered the flight path with exactly the same initial energy, then very compact (highly focused) partial parcels each consisting of ions of identical mass would be created. In practice there is a range of kinetic energies initially imparted to the ions within a partial parcel and this can lead to a range of flight times of ions within any given partial parcel that is broad enough to overlap flight time ranges of adjacent partial parcels.
The solution to this problem has been to provide a focusing deflection field in the flight path. The deflection field causes the partial parcels to traverse one or more arcs. In so doing, within each partial parcel the ions of higher kinetic energies in undergoing the same angular deflection traverse arcs of longer radii than ions of lower kinetic energies. Thus, the time required for ions of differing kinetic energies within each partial parcel to traverse the deflection field is evened out by the unequal arc paths. By locating the deflection field between time measurement reference locations in the flight path, usually referred to as entrance and exit planes, the result is to focus the partial parcels. Stated another way, the function of the deflection field is to make the flight time of ions in each partial parcel a function of the ratio of ion mass (m) to charge (e) rather than initial differences in kinetic energies.
A schematic diagram of a conventional time of flight mass spectrometer containing a deflection field is shown in FIG. 1. The mass spectrometer 100 is comprised of a central vacuum chamber 102 defining an ion flight path indicated by arrows 104 extending between an entrance plane 106 and an exit plane 108. The ambient pressure in the vacuum chamber is maintained below 1.33.times.10.sup.-4 kilopascals (&lt;10.sup.-5 torr) to minimize ion collisions with the ambient atmosphere. There is located in the vacuum chamber between the dashed lines 110 and 112 a deflection field zone 114. The deflection field as shown is a preferred quadruple focusing deflection field, more specifically described below, but the deflection field in its simplest form can deflect the ions in their flight path through a single arc. A pulsed ion source 116 emits a parcel of accelerated ions across the entrance plane into the flight path within the vacuum chamber. The ion source is also internally evacuated and can therefore be viewed as an extension of the flight path vacuum chamber. Beyond the exit plane there is located a receiving unit 118 for the ions traveling along the flight path. The receiving unit forms a second extension of the ion flight path vacuum chamber. By referencing the time at which receipt of a partial parcel is detected to the time a target pulse was generated in the ion source, a measurement of the time elapsed in traversing the flight path vacuum chamber between its entrance and exit planes can be provided.
The Problems to be Overcome
The problems which the present invention specifically address are the loss of ions from the flight path in the deflection field and the inability of conventionally constructed deflection fields to bring the ions back into focus.
To appreciate the problems and the novel solutions provided by this invention it is necessary to review the construction of conventional deflection fields. Deflection fields are formed by one or more pairs (usually four pairs) of spaced inner and outer electrodes. A typical electrode pair arrangement is shown in FIG. 2. The inner electrode 201 provides an ion guiding surface 203 which is cylindrical in shape over an arc of approximately 270.degree.. This inner ion guiding surface has a radius R.sup.1. Spaced from the inner electrode is an outer electrode 205 providing an outer ion guiding surface 207 which is cylindrical in shape over the same approximately 270.degree. arc. The outer ion guiding surface has a radius R.sup.2. Both radii R.sup.1 and R.sup.2 have a common origin C.
In operation, ions traveling along a linear flight path L enter the space S between the inner and outer electrodes. The ions in the flight path all exhibit the same charge polarity. In addition they exhibit a range of kinetic energies above and below an average value. The inner and outer electrodes are electrically biased to exhibit the same polarity as the ions. The voltage applied to the outer electrode is higher than that applied to the inner electrode. The voltages can be selected by known relationships to allow ions of average kinetic energy to traverse the arc defined by the spaced electrodes along a flight path mid-way between the opposed inner and outer ion guiding surfaces. The ions are deflected and guided by charge repulsion. Ions of slightly higher than average kinetic energies must approach the outer ion guiding surface somewhat more closely to be repelled and therefore traverse an arc of a slightly longer than average radius. Conversely, ions of slightly lower than average kinetic energies are repelled from the outer electrode ion guiding surface more readily and traverse an arc having a somewhat shorter than average radius.
Since in FIGS. 1 and 2 only the main ion flight paths are schematically illustrated, it must be borne in mind that in practice some ions, from the time they pass through the aperture in the accelerating electrode, diverge from the desired flight path. This is best illustrated by FIG. 3, which is a schematic sectional view taken along the flight path L. The radial vectors V schematically represent (on an exaggerated scale) the radial components of the flight of individual ions. When the vectors V are combined with the flight vectors along the flight path L, it can be appreciated that the ions in flight lie within a cone of scatter of which the flight path L is the idealized embodiment when V is zero.
To the extent that ions diverge from the ideal flight path L they can fail to reach the partial parcel detector. This results in signal strength reduction, thereby increasing the demands that must be placed on both the source and detection means to compensate for this loss.
As shown in FIG. 4, when the ions are traveling between the ion guiding surfaces of the inner and outer electrodes, ion divergence is partially repressed by the field gradient between the electrodes. However, when the radial vector of flight of a ion lies in a plane of uniform potential--i.e., any vertical plane, divergence of the ion is not overcome, as schematically indicated by vectors V.sup.1 and V.sup.2. From FIG. 4 it is apparent that the cylindrical ion guiding surfaces can repress lateral divergence of the ions from their ideal flight path, thereby spatially focusing the ions in one spatial dimension, but are ineffective to achieve complete focusing of the ions in the ideal flight path L.
A modified construction of deflection field electrodes that has been described in the art is shown in FIGS. 5 and 6. Field plates 209 and 211 are mounted above and below the inner and outer electrodes. The field plates are also electrically biased to repel the ions, but since they are maintained at a potential different from both that of the inner and outer electrodes, they are spaced from both electrodes. Further, since the inner and outer electrodes are themselves at differing potentials, the field plates must be spaced to avoid shorting these electrodes.
The deficiencies of the field plates is schematically shown in FIG. 6, which is a section taken along section lines 6--6 i FIG. 2. Ions which diverge from flight path L along flight paths L.sup.1 and L.sup.2 which include the vectors V.sup.1 and V.sup.2, respectively, as components, are repelled by the field plates 209 and 211, but are not returned to the flight path L. Instead they are free to escape from the deflection field through the spacing between the field plates and and the outer electrode. Thus, the addition of field plates does not overcome the problem of loss of ions from the flight path.
Prior Art
The following are illustrative of the prior state of the art:
R-1 Poschenrieder, "Multiple-Focusing Time of Flight Mass Spectrometers Part I. TOFMS With Equal Momentum Acceleration", International Journal of Mass Spectrometry and Ion Physics, Vol. 6, 1971, pp. 413-426.
R-2 Poschenrieder, "Multiple-Focusing Time of Flight Mass Spectrometers Part II. TOFMS With Equal Energy Acceleration", International Journal of Mass Spectrometry and Ion Physics, Vol. 9, 1972, pp. 357-373.
R-3 Poschenrieder U.S. Pat. No. 3,863,068, issued Jan. 28, 1975.
R-4 Sakurai et al, "Ion Optics for Time-of-Flight Mass Spectrometers with Multiple Symmetry", International Journal of Mass Spectrometry and Ion Processes, Vol. 63, 1985, pp. 273-287.
R-5 Sakurai et al, "A New Time-of-Flight Mass Spectrometer", International Journal of Mass Spectrometry and Ion Processes, Vol. 66, 1985, pp. 283-290.
R-6 Sakurai et al, "Particle Flight Times in a Toroidal Condenser and an Electric Quadrupole Lens in the Third Order Approximation", International Journal of Mass Spectrometry and Ion Processes, Vol. 68, 1986, pp. 127-154.